1. Field of the Invention
The present invention relates to a lens control apparatus for controlling a lens moving parallel to an optical axis in an inner focus type lens system. Related Art of the Invention
2. Related Background Art
FIG. 1 is a view showing the structure of an inner focus type lens system. An inner focus type lens system 1 includes a first fixed lens 2, a magnification lens 3, an iris 4, a second fixed lens 5, and a focus compensation lens 6, all of which are sequentially arranged from the left object side to the right side along the optical axis. The magnification lens 3 is moved parallel to the optical axis to perform magnification. The focus compensation lens 6 has a focus control function upon parallel movement along the optical axis and a so-called compensation function of correcting movement of a focal plane upon magnification. An optical object image obtained by the lens system 1 is focused on an image pickup surface 7a of an image pickup element 7 and is photoelectrically converted into a video signal.
In the lens system 1 having the arrangement described above, if the focal length remains the same, the position of the focus compensation lens 6 for focusing an object image on the image pickup surface 7a of the image pickup element 7 varies depending on object distances because the focus compensation lens 6 has both the compensation function and the focus control function.
When the object distance is changed at the respective focal lengths, and the positions of the focus compensation lens 6 for focusing object images on the image pickup surface 7a are continuously plotted, the result is obtained, as shown in FIG. 2. Under magnification, a locus (FIG. 2) corresponding to an object distance is selected. When the focus compensation lens 6 is moved in accordance with the selected locus, zooming free from blurring can be performed.
In a front-element focus type lens system, a compensation lens independently of a magnification lens is arranged, and the magnification lens is coupled to the compensation lens through a mechanical cam ring. For example, when a manual zoom knob is attached to this cam ring to manually change the focal length, the cam ring can follow the quick movement of the knob, and the magnification lens and the compensation lens are moved along the cam groove of the cam ring. If the focus lens is set in a focused condition, blurring will not occur.
In control of an inner focus type lens system having the above characteristic feature, a plurality of pieces of lens locus information shown in FIG. 2 are stored in a lens control microcomputer in any form. A proper lens locus is selected in accordance with the positions of the magnification lens 3 and the focus compensation lens 6. Zooming is thus generally performed in accordance with the selected locus.
The position of the focus compensation lens 6 with respect to the position of the magnification lens 3 is read out from a storage element to control the lenses 3 and 6. Read access of the positions of the lenses 3 and 6 must be performed with high precision. In particular, as can be apparent from FIG. 2, when the magnification lens 3 is moved at a constant speed or a speed close thereto, the inclination of the locus of the focus compensation lens 6 is instantaneously changed in accordance with a change in focal length. This indicates that the speed and orientation of the movement of the focus compensation lens 6 are instantaneously changed. In other words, the actuator for the focus compensation lens 6 must have a high-precision speed response of 1 Hz to several hundreds of Hz.
As a focus compensation lens drive actuator in an inner focus type lens system which satisfies the above requirement, a stepping motor is generally used. This stepping motor rotates in perfect synchronism with a stepping pulse output from the lens control microcomputer. High speed response precision and stop precision, and positional precision can be obtained because a stepping angle per pulse is predetermined.
The stepping pulses for the stepping motor can be used for an increment type position encoder because a rotation angle corresponding to a stepping pulse count is predetermined. Any special position encoder need not be used.
As described above, when a magnification operation is to be performed using a stepping motor while maintaining a focused condition, the locus information in FIG. 2 must be stored in the lens control microcomputer or the like in any form (i.e., a locus itself or a function using a lens position as a variable), and proper locus information is read out in correspondence with a given position or moving speed of the magnification lens. The focus compensation lens must be moved on the basis of the readout locus information.
FIG. 3 is a view for explaining a locus tracking method proposed prior to the present invention. Referring to FIG. 3, the focus compensation lens position is plotted along the ordinate, and the magnification lens position is plotted along the abscissa. Positions z.sub.0, z.sub.1, z.sub.d 2, . . . z.sub.11 represent magnification lens positions, and loci a.sub.0, a.sub.1, a.sub.2, . . . , a.sub.11, and loci b.sub.0, b.sub.1, b.sub.2, . . . , b.sub.11 represent typical lens loci stored in the lens control microcomputer. Loci p.sub.0, p.sub.1, p.sub.2, . . . , p.sub.11 represent lens loci calculated on the basis of the above two different loci stored in the lens control microcomputer. The calculation equation of this lens locus will be described below: EQU p.sub.(n+1) =.vertline.p.sub.(n) -a.sub.(n).vertline./.vertline.b.sub.(n) -a.sub.(n).vertline..times..vertline.b.sub.(n+1) -a.sub.(n+1).vertline.+a.sub.(n+1) (1)
According to equation (1), when the focus compensation lens is located on the locus p.sub.0 in FIG. 3, the locus p.sub.0 calculates a ratio which interpolates a line segment b.sub.0 -a.sub.0, and a point which interpolates a line segment b.sub.1 -a.sub.1 is defined as p.sub.1 in accordance with the resultant ratio.
In this case, however, when the magnification lens position is not located on a zoom boundary (i.e., any of the positions z.sub.0, z.sub.1, . . . , z.sub.11 in FIG. 3), i.e., when the magnification lens position and the focus compensation lens positions are given as Z.sub.x and P.sub.x, respectively, the locus tracking position is not updated. For example, when the focus compensation lens position is changed from P.sub.x to Q.sub.x in FIG. 3 in accordance with AF (auto-focus) information changing in correspondence with a change in object distance in zooming in the AF mode, the locus tracking position is not immediately updated to cause blurring. When the moving speed of the magnification lens increases as in high-speed zooming, a period for causing the magnification lens position to update a zoom zone on a zoom boundary (i.e., a time required to move the magnification lens from Z.sub.(n) to Z.sub.(n+1) is shorter than the period of local tracking position calculation of the microcomputer. For this reason, the locus tracking position cannot be updated on all the zoom boundaries. As a result, blurring frequently occurs.
In the above case, when the magnification lens position is not located on the zoom boundary (i.e., any of the positions z.sub.0, z.sub.1, z.sub.2, . . . , z.sub.11 in FIG. 3), i.e., when the magnification lens position and the focus compensation lens position are Z.sub.x and P.sub.x, respectively, locus data is not available in the lens control microcomputer. In this case, positions a.sub.x and b.sub.x in FIG. 3 must be calculated, and p.sub.(n+1) must be obtained by substituting a.sub.(n) =a.sub.x and b.sub.(n) =b.sub.x, and p.sub.(n) =p.sub.x into conventional equation (1). Processing thus becomes complicated, and calculation errors may be accumulated. As a result, trouble occurs in zooming in the focused condition.